Heretofore, in photolithography, it is common to employ an exposure device to transfer a fine circuit pattern onto a wafer to produce an integrated circuit. Along with high integration and high functionality of integrated circuits, microsizing of integrated circuit has been progressing, and an exposure device is required to form an image of a circuit pattern on a wafer with a high resolution in a deep focal depth, whereby blue shift of the exposure light source is in progress. The exposure light source has been advanced from the conventional g-line (wavelength: 436 nm), i-line (wavelength: 365 nm) or KrF excimer laser (wavelength: 248 nm), and now an ArF excimer laser (wavelength: 193 nm) is being used. Further, in order to be prepared for an integrated circuit for the next generation where the line width of a circuit pattern will be less than 100 nm, it is considered to be prospective to employ a F2 laser (wavelength: 157 nm) as the exposure light source, but it is considered that even this can not cover beyond a generation of a line width of 70 nm.
Under these circumstances, a lithographic technique employing typically a light having a wavelength of 13 nm among EUV light (extreme ultraviolet light) as the exposure light source, has attracted attention, as it may be applied to the printing of feature sizes of 50 nm and smaller. The image-forming principle of the EUV lithography (hereinafter referred to as “EUVL”) is the same as the conventional photolithography to such an extent that a mask pattern is transferred by means of an optical projection system. However, in the energy region of EUV light, there is no material to let the light pass therethrough. Accordingly, a refraction optical system can not be used, and an optical system will be required to be a reflection optical system in all cases.
The optical material for the exposure device to be used for EUVL will be a photomask, a mirror or the like, and it is basically constituted of (1) a base material, (2) a reflective multilayer formed on the base material and (3) an absorber layer formed on the reflective multilayer. For the multilayer, it is studied to form layers of Mo/Si alternately, and for the absorber layer, it is studied to use Ta or Cr as the layer-forming material. As the base material, a material having a low thermal expansion coefficient is required so that no strain will be formed even under irradiation with EUV light, and a glass having a low thermal expansion coefficient is being studied.
TiO2—SiO2 glass is known to be a very low thermal expansion material having a coefficient of thermal expansion (CTE) smaller than quartz glass, and the coefficient of thermal expansion can be controlled by the TiO2 content in the glass, whereby it is possible to obtain a zero expansion glass having a coefficient of thermal expansion being close to zero. Accordingly, TiO2—SiO2 glass is prospective as a material to be used for an optical material for the exposure device for EUVL.
In a conventional method for preparing TiO2—SiO2 glass, firstly, a silica precursor and a titania precursor are, respectively, converted into a vapor form, and then mixed. Such a vapor form mixture is feeded into a burner and thermally decomposed to form TiO2—SiO2 glass particles. Such TiO2—SiO2 glass particles will be deposited in a refractory container and at the same time will be melted to form TiO2—SiO2 glass. However, TiO2—SiO2 glass prepared by this method has had a periodical fluctuation of the TiO2/SiO2 ratio, which appears as striped striae with a 100 to 200 μm pitch.
Further, U.S. patent application publication No. 2002/157421 discloses a method which comprises forming a TiO2—SiO2 porous glass body, converting it to a glass body, and then obtaining a mask substrate therefrom.
The striped striae of the TiO2—SiO2 glass are considered to form as the difference in the refractive index increases due to the periodical fluctuation of the TiO2/SiO2 ratio in the glass material. When it is to be used as an optical material for the exposure device for EVUL, the TiO2—SiO2 glass is required to be polished so that the glass will have an ultra smooth surface. However, in the TiO2—SiO2 glass, at the portion where the TiO2/SiO2 compositional ratio is different, the mechanical and chemical properties of the glass vary depending upon the composition, whereby the polishing rate tends to be non-uniform, and it is difficult to finish so that the glass surface after polishing will be ultra smooth. Further, if a TiO2—SiO2 glass having striped striae with a 100 to 200 μm pitch, is polished, “waving” will be formed on the glass surface with a pitch of the same degree as the striae pitch, whereby it is very difficult to obtain ultra smooth surface.
In recent years, it has been pointed out that it is necessary to reduce MSFR (Mid-Spatial Frequency Roughness) having a waving pitch of from 10 μm to 1 mm, as an extremely important characteristic required for an optical material for the exposure device for EUVL. When a conventional TiO2—SiO2 glass is polished, it has a waving with a 100 to 200 μm pitch for the above-mentioned reason, whereby it has been very difficult to reduce MSFR.
Accordingly, in order to finish so that the glass surface after polishing will be ultra smooth, as an optical material for the exposure device for EUVL, it is considered effective to minimize the fluctuation of the TiO2/SiO2 ratio of the TiO2—SiO2 glass to make the polishing rate constant at the glass surface, and to reduce the striae pitch to a level of at most 10 μm to reduce MSFR.
Further, even with a TiO2—SiO2 glass substrate having the same level of smoothness (Roughness (rms)), rough surface of a glass whose striae pitch is smaller can be polished efficiently in a short time, and can be finished to be ultra smooth easily.
Further, it is important to make the TiO2/SiO2 ratio uniform in the TiO2—SiO2 glass, with a view to minimizing the fluctuation of the coefficient of thermal expansion within the glass. Accordingly, in addition to minimizing the fluctuation of the TiO2/SiO2 ratio in the small areas so-called striae, it is preferred to minimize the fluctuation of the TiO2/SiO2 ratio in the entire region of the material.